1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to performing a calibration to improve longitudinal balance in a circuit.
2. Description of the Related Art
In communications systems, particularly telephony such as a Plain Old Telephone System (POTS), it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The functions of the line card include supplying talk battery, performing wake-up sequences of circuits to allow communications to take place, and the like. Voltage signals are processed and conditioned when being driven onto telecommunication lines.
POTS was designed primarily for voice communication, and thus provides an inadequate data transmission rate for many modern applications. To meet the demand for high-speed communication, designers have sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological solutions proposed in the telecommunications industry use the existing network of telephone wires. A promising one of these technologies is the Digital Subscriber Line (xDSL or DSL) technology.
xDSL is making the existing network of telephone lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit video, television, and very high-speed data. The fact that more than six hundred million telephone lines exist around the world is a compelling reason for these lines to be used as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
There are now at least three popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, all four DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein.
DSL and POTS technologies can co-exist in one line (e.g., also referred to as a “subscriber line”). Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels, from 25 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data. It should be noted that the standards for certain derivatives of ADSL are still in definition as of this writing, and therefore are subject to change.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate typically found in conventional POTS systems.
DSL systems generally employ a signal detection system that monitors the telephone line for communication requests. More specifically, the line card in the central office polls the telephone line to detect any communication requests from a DSL data transceiver, such as a DSL modem, located at a subscriber station. There are multiple types of signals that are received and transmitted over multiple signal paths during telecommunication operation. Many times, feedback configurations in the amplifiers that process the transmission signals cause noise and power problems.
Many of today's amplifier circuits may call for electronic components that have high bandwidth, which may increase consumption of power. Many times, power consumption in the line card can become undesirably high. Amplifier circuits that are used to condition communication signals often consume large amounts of power. Excessive power use can compromise the effectiveness of line cards, particularly for remote line cards, which rely upon portable power supplies. Excessive power consumption can also require additional resources to counteract the effects of high power consumption, such as additional cooling systems to keep line card circuitry in operating condition. Excessive power consumption can also require additional circuits to furnish the required amounts of power needed for efficient operation of line cards. Excessive power consumption can cause appreciable inefficiencies in the operation of line cards and the communication system as a whole.
The prior art implementations of signal conditioning circuits generally implement signal feedback configurations that generally take the output voltage signal and then feed the voltage signal back to a negative input of an amplifier within a circuit. In other words, the direct output voltage signal is the feedback signal used in the implementations described above. Among the problems associated with the current implementations, include the fact that a larger signal is fed back into the circuit described above. The problem with such an implementation is that larger signals may generally carry larger amounts of noise. Therefore, feeding back larger signals amounts to feeding back larger amounts of noise into the circuit, which may cause performance problems in the amplifier circuit.
Additionally, feeding back the output voltage signal may require amplifiers that have relatively large bandwidth capabilities. Utilizing amplifiers with larger bandwidth capabilities generally increases power consumption. Additionally, many prior art systems employ feedback configurations that use single-ended feedback signals. The circuits that use these types of configurations may experience an excessive amount of longitudinal signals, such as longitudinal currents. Longitudinal signals may enter one or more amplification stages within a circuit and cause excessive noise. Utilizing the current methodologies, the performance of a signal conditioning circuit may be compromised.
Often, longitudinal imbalance may occur in circuits that employ multiple signals or differential-type amplifiers. For example, telecommunication signals generally comprise a ring signal and a tip signal. The forward paths of the tip and ring signals may contain longitudinal imbalance that may cause signal degradation or errors in telecommunications. Longitudinal balance may be dominated by the differential gain between the tip and ring forward paths. The common mode error that exists on the differential-type circuits can cause significant errors in telecommunications.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.